1. Field of the Invention
This invention relates generally to electrochemistry and preventing galvanic corrosion in piping systems that convey fluid electrolytes through adjoining pipes constructed of dissimilar metals and more particularly relates to a control system for a galvanic corrosion inhibiting pipe coupling.
2. Description of the Related Art
Seawater piping systems on ships and on shore facilities frequently contain dissimilar metal interfaces that pose galvanic corrosion problems. The corrosion is the result of the oxidation that occurs at the more active metal when a relatively more noble metal and a relatively more active metal are in electrical contact with an electrolyte and are conductively connected together through an external circuit. It is desirable that this galvanic corrosion be minimized to reduce the life cycle costs of these systems. One promising technology to mitigate galvanic corrosion in dissimilar metal piping systems is the galvanic corrosion inhibiting coupling that is described in my U.S. Pat. No. 5,739,424 which is herein incorporated by reference.
The galvanic corrosion inhibiting coupling described in the patent is mounted between the ends of two dissimilar metal pipes. The tubular interior walls of the pipes and the coupling define the fluid conveying chamber. Longitudinally spaced, annular electrodes are mounted in grooves formed in the tubular interior wall of the coupling. These protection electrodes, which are sometimes also referred to as bi-electrodes, are in electrical contact with the fluid electrolyte within the chamber. A current source is electrically connected to the protection electrodes through penetrations in the sidewall of the coupling. An electrical current is generated through the electrolyte between the protection electrodes creating an ohmic potential drop in the fluid. This potential drop in the fluid minimizes the galvanic corrosion potential shift of the pipes from the native potential that would exist in the absence of the dissimilar metal couple. One of the protection electrodes, with positive polarity, functions as the anode and the other protection electrode, with negative polarity, functions as the cathode in the galvanic corrosion inhibiting coupling. When the voltage applied to the protection electrodes creates a sufficient ohmic potential drop in the fluid between the dissimilar pipes, each pipe “sees”, in the direction of the other pipe, a potential substantially equal to its own native potential. Therefore, the potential difference of each galvanic cell is greatly reduced resulting in greatly reduced galvanic current.
It is desirable to automatically adjust the voltage applied to the protection electrodes during the operating lifetime of the galvanic corrosion inhibiting coupling because scale build up and the development of protective oxides on the interior walls of the pipes and variations in the electrolyte within the pipes cause variations of the potentials of the two dissimilar pipes. Consequently, the ohmic potential drop in the electrolyte that results from the applied voltage is desirably adjusted to compensate for variations in the potentials of the pipes in order to maintain maximum protection.
The above patent suggested that the voltage applied to the protection electrodes be controlled using a pair of sensor electrodes that are located near the ends of the coupling, one sensor electrode near each of the dissimilar pipes. The sensor electrodes sought to continuously sense the potential of each of the two pipes by means of their close proximity to those pipes. In that arrangement, the voltage applied to the protection electrodes was adjusted as a function of the voltage difference between the pair of sensor electrodes. The voltage applied to the protection electrodes was made proportional to the sensed potential difference between the pipes.
However, this system of sensing the potential difference between two sensor electrodes as an approximation of the pipe potential difference has been found inadequate. The rate of galvanic corrosion is a function of the galvanic current flowing through the electrolyte between the dissimilar metal pipes.
In the original demonstration work described in my above patent, in association with FIG. 7 of that patent, the galvanic current flowing between the dissimilar metal pipes was measured directly in the external circuit and this external return current could be used to demonstrate the effectiveness of the concept. However, on a shipboard or on-shore application, all of the piping typically is grounded together to a common ground at multiple locations along the piping to minimize the risk of electrical shock and to prevent static charge build-up. This multi-location grounding creates multiple paths for the galvanic current flow in the external circuit. Consequently, the external circuit current can not be monitored in a practical manner. Thus, the galvanic current is not directly measurable in actual shipboard or shore implementations of the invention in my above patent.
It is therefore an object and feature of the invention to provide a control circuit and method for automatically controlling the galvanic corrosion inhibiting coupling described in my above patent.